Exactly this, it's basically GTA5 private servers with fully customised game mechanics and strict rules around conduct where you're playing a character in a realistic setting, like an actor of sorts. It's big money for the people who run these servers too, especially NoPixel which is the most popular streamer server and difficult to get whitelisted on.

Rule breaks, for example not fearing for your life and acting appropriately when in a dangerous situation are potentially risking a ban. There is a full set of laws and police, judges, lawyers and a whole court and prison system modded into to deal with criminals with players roleplaying all of these roles. There's a hospital, ambulances, doctors and you have to get someone else to call 911 to have a paramedic drive out and rescue you if you die.

It's honestly impressive how much and well modders have modded GTA5 over the years. If you wanna try it yourself download the FiveM client and there are hundreds of servers you can join and play on.

Be warned though, just like older games with private servers (like old Battlefield/CS ect games) you are ultimately at the mercy of whoever the admins who run them are and sometimes these can be interesting individuals. Rockstar recently bought out the FiveM platform but they have no real control over what these private individuals do short of banning the whole server from the platform which rarely happens.